Recently, cleaning of products in a semiconductor device plant, and a factory for producing liquid crystal electronic components and the like has been becoming increasingly sophisticated with complication of production processes, and fine circuit patterns. For example, fine particles, metal, organic matter and the like adhered to a silicon wafer are removed by use of a special liquid (referred to as a cleaning solution) prepared by dissolving high-purity gas or high-purity gas and chemicals in a functional water (such as ultrapure water).
As a cleaning processing system, a “sheet processing system” for performing chemical cleaning and ultrapure water cleaning for each wafer in accordance with products of small-volume production in great varieties is employed in addition to a “batch processing system” for repeating soaking operation and cleaning operation of a plurality of silicon wafers at the same time. Cleaning process time (tact time) per wafer in the sheet processing system is longer than cleaning process time per wafer in the batch processing system, and the used amount of a cleaning solution per wafer is large, and therefore shortening of the tact time and reduction of the used amount of the cleaning solution are requested. In the present situation, in order to perform effective cleaning for a short time, and reduce the used amount of the cleaning solution, an advanced cleaning process for singly or simultaneously using a plurality of functional waters and chemicals, and switching a cleaning process for a short time is performed.
As the functional water, for example, ozone water prepared by dissolving ozone gas in ultrapure water is used. Ozone dissolved in ultrapure water has extremely strong oxidizability even at low concentration (several ppm), and therefore can remove organic matter or metal. This ozone water is generally produced by an ozone-water producing apparatus. With increase in sophistication and complication of the cleaning process, ozone water supply and stop to a cleaning apparatus for a short time are required. However, when a conventional apparatus stops production of ozone water once, a certain amount of time (rise time) is required until ozone water can be supplied at required ozone concentration and at a required flow rate again. Therefore, in order to respond to supply request of ozone water to the cleaning apparatus, the ozone-water producing apparatus regularly produces ozone water, and continuously supplies the produced ozone water to the cleaning apparatus. As a result, an excess amount of ozone water is supplied to the cleaning apparatus, and unused ozone water which is not used for cleaning of a silicon wafer is discharged as waste water from the cleaning apparatus.
Conventionally, there is proposed a circulation type ozone-water supplying apparatus capable of supplying ozone water having constant concentration and constant pressure, and recycling unused ozone water regardless of the used amount of the ozone water at a use point (refer to Patent Literature 1).
In the conventional circulation type ozone-water supplying apparatus, as illustrated in FIG. 6, water and ozone gas are supplied to an ozone dissolving tank 12 to produce ozone water, supplies the ozone water from the ozone dissolving tank 12 to a circulation tank 21, supplies the ozone water from the circulation tank 21 to a use point through an ozone-water supply pipe 22, returns the ozone water which is not consumed at the use point to the circulation tank 21 through an ozone-water return pipe 23, and supplies the ozone water from the circulation tank 21 to the use point again. Then, the in-tank pressure of the ozone dissolving tank 12, the in-tank pressure of the circulation tank 21, and the in-pipe pressure of the ozone-water return pipe 23 are maintained constant, and the in-tank pressure of the circulation tank 21 is controlled to pressure lower than each of the in-tank pressure of the ozone dissolving tank 12 and the in-pipe pressure of the ozone-water return pipe 23.